Proximity sensor

ABSTRACT

An immersion sensor for use with a cushion or mattress for determining the relative immersion of a person within the cushion or mattress comprising a sensor, a ground and a circuit for measuring capacitance. The sensor comprises a sheet of conductive material, and the ground comprises a second sheet of conductive material. The circuit is adapted to send short bursts of electrical current to the sensor and a capacitor. The circuit is further adapted to measure the length of time the burst of current takes to charge the capacitor. Based upon the measured time, the circuit calculates the proximity of the object based upon the time taken to charge the capacitor. A method that may be implemented with the immersion sensor is also disclosed.

RELATED APPLICATIONS

The present application is a continuation of application Ser. No.11/412,859, filed Apr. 27, 2006, which claims priority to U.S.Provisional Application Ser. No. 60/725,901 filed Oct. 12, 2005, U.S.Provisional Application Ser. No. 60/725,006 filed Oct. 6, 2005, and alsoU.S. Provisional Application Ser. No. 60/675,315 filed Apr. 27, 2005.The contents of said applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to proximity sensors. More specifically, theinvention relates to a sensor for detecting a relative distance of anobject to the sensor by detecting changes in charge transfer.

BACKGROUND OF THE INVENTION

Proximity sensors for detecting an actual or relative distance betweenthe sensor and an object are known in the art. For example, U.S. Pat.No. 6,621,278 to Arie Ariav discloses a method of measuring a distanceby transmitting a cyclically-repeating wave. The wave is then receivedat a second location in the medium. The system detects a predeterminedpoint in the cyclically-repeating wave that is received at the secondlocation and continuously changes the frequency of transmission of thecyclically-repeating energy wave in accordance with the detected pointof each received cyclically-repeating wave received at the secondlocation such that the number of waves received at the second locationis a whole integer. The change in frequency to produce a measurement ofthe predetermined parameter is used to determine the distance the wavehas traveled. However, this system has drawbacks, particularly in thatthe sensor is unduly complex both in electronic implementation and insensor construction.

Other types of detectors, primarily for detecting the presence orabsence of an object, use ultrasonic and radio frequency transmittersand detectors that receive reflected energy when an object is present inan area of interest. These detectors however cannot be used practicallyto detect a relative or actual distance, particularly in very shortdistances. In certain settings, the amount of RF energy generated bythese types of device is unacceptable due to interference. Moreover,some people have concerns about constant exposure to RF energy.

Many applications require low power consumption and detection of arelative distance within a range of interest. For example, cushions forwheelchairs must be inflated to a pressure that is sufficient toproperly immerse the person in the cushion to prevent the formation ofdecubitus ulcers on the person in the wheelchair. However, often thepeople bound to the wheelchair do not have the ability to feel when theyare properly immersed in the cushion, such as a paraplegic orquadriplegic person. For those people, others must periodically checkthe person's immersion within the cushion to ensure the person is not inan overinflated state, such that only a small portion of the person'sbody is bearing their weight, or in an underinflated state, such thatthe person has “bottomed out” and is no longer supported entirely by thecushion. Similarly in a cushion not inflated with air, problems alsoexist when determining the proper cushion immersion. However, presently,no acceptable means of detecting the immersion of a person in a cushionexists. Only indirect measurement of pressure internally in the cushionis available. This type of measurement is dependant upon the materialsof construction and structural conformation all creating significantlimitations in the applicability of the measurement.

Likewise, people bound to hospital beds must avoid decubitus ulcers whenconfined to the bed for long periods of time. To accomplish this,inflation mattresses are commonly used, and the inflation level of themattress must be monitored in order to maintain the proper inflationlevel to prevent over inflation or under inflation of the mattress.Moreover, because the person's weight is concentrated over their entireback side, multiple locations must be checked for under inflation orover inflation. As a result, a sensor which is divided into zones tocheck the immersion of the patient within the mattress is needed.

SUMMARY OF THE INVENTION

The present invention comprises an immersion sensor for use with acushion or mattress for measuring the depth of immersion of a personwithin the cushion or mattress comprising a sensor, a ground and/orshield and a circuit for measuring capacitance. The sensor comprises asheet of conductive material, and the ground comprises a second sheet ofconductive material. The circuit is adapted to send short bursts ofelectrical current to the sensor and the reference capacitor. Thecircuit is further adapted to measure the length of time the burst ofcurrent takes to charge the capacitor. Based upon the measured time, thecircuit calculates the proximity of the object based upon the time takento charge the capacitor. The present invention also comprises a methodthat may be implemented with the immersion sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a wheelchair cushion proximitydetection device according to an embodiment of the present invention;

FIG. 2 is a plan view of the conductive and nonconductive layers of theproximity detection device according to an embodiment of the presentinvention;

FIG. 3 is a diagram of a circuit according to an embodiment of thepresent invention;

FIG. 4 is a diagram of a circuit of a charge transfer device accordingto an embodiment of the present invention;

FIGS. 5A-5D are a flow chart showing the operation of the hardware andsoftware of the circuit of FIG. 3;

FIG. 6 is an exploded perspective of a proximity detector for a bed aircushion according to an embodiment of the present invention;

FIG. 7 is a diagram of a circuit according to another embodiment of thepresent invention;

FIG. 8 is a diagram of a sensor placement on a bed cushion proximitydetector according to an embodiment of the present invention;

FIG. 9 is a diagram of a circuit according to another embodiment of thepresent invention;

FIG. 10 is a diagram of a sensor placement on a bed cushion proximitydetector according to yet another embodiment of the present invention;

FIG. 11 is a diagram of a sensor placement on a bed cushion proximitydetector according to yet another embodiment of the present invention;

FIG. 12 is an exploded perspective view of an automatically adjustingwheelchair cushion according to an embodiment of the present invention;

FIG. 13 is a perspective view of an embodiment of the device including afirst sensor of relatively large area and a second sensor of relativelysmall surface area according to an embodiment of the present invention;

FIG. 14 is a perspective view of an embodiment of the device including afirst sensor of relatively large area and a second sensor of relativelysmall surface area with a ground plane according to an embodiment of thepresent invention;

FIG. 15 is a diagram of a circuit for operating the embodiment of FIG.14;

FIG. 16 is diagram of an embodiment of the present invention including avisual display device; and

FIG. 17 is a diagram of a sensor placement on a bed cushion proximitydetector according to yet another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail preferred embodiments of the invention with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit the broadaspect of the invention to the embodiments illustrated.

The preferred embodiment of the present invention is a proximity sensorthat utilizes charge transfer measuring technology and large-areacapacitive sheets to determine the distance of an object from thecapacitive sheet. The charge transfer measurement is employed with ashort, low duty cycle burst of power. Burst mode permits powerconsumption in the low microamp range, thereby dramatically reducesradio frequency (RF) emissions, lowers susceptibility to electromagneticinterference (EMI), and yet permits excellent response time. Internally,it is preferred that the signals are digitally processed to generate therequired output signals. The charge transfer measurement device switchesand charge measurement hardware functions are preferably all internal tothe charge transfer measurement device.

To that end, the invention will be described, by way of example and notby limitation, in reference to a cushion for a wheelchair. Referring toFIG. 1, there is shown an inflatable cushion 10, for example the cushiondescribed in U.S. Pat. No. 4,541,136. Placed below the cushion is asensor 12 according to the present invention to detect the immersion ofa person within the cushion. The sensor 12 comprises two exterior sheetsof neoprene rubber 14. Sandwiched between the sheets of rubber are thinlayers of foam 16 and between the foam 16 is a sensor layer 18.

The sensor layer 18 of FIG. 2 comprises a conductive sheet 20 adhered toa nonconductive sheet 22. The conductive sheet 20 is preferably madefrom copper, and the nonconductive sheet 22 is preferably made from apolyester film. The sensor layer 18 may also be made from any otherconductive material, such as a conductive polymer. The conductive sheet20 when made from copper preferably has a thickness of about 0.0005 ofan inch. The conductive sheet 20 is interrupted, preferably by etchingor die cutting, along an area 24 to form a sensor area 26 and agrounding plane area 28. While the sensor layer 18 is described ascopper and polyester sheets, the nonconductive sheet is not required andmay be omitted and the conductive sheet may be made from any conductivematerial, such as a conductive braid, mesh or screen printing aconductive material onto a nonconductive base. Additionally, while thesensor area 24 is shown as rectangular, the sensor area 26 may beappropriately shaped and located in order to provide the optimumgeometry to the object to be sensed. In the example of FIG. 2, thesensor area is confined to a rear portion of the sensor where a person'sbuttocks would be located when seated in the wheelchair. Since most of aperson weight is distributed in this location while seated, this locatedis at the greatest danger of bottoming out. However, it is within thescope of the present invention to provide a sensor at any location ormultiple locations of the seating area.

The problem solved by the ground layer with using charge transfer orcapacitive technology with wheel chair cushions is that there is no goodground to use as reference. The grounding plane area 39 being in thearea around the sensor area 26 allows a capacitance measurement to bemade relative to the distance between the person and the sensor andground areas 26 and 28. The present invention is attached to a circuit30 as shown in FIG. 3. The circuit generally comprises a microcontroller32, such as a 16LF818 available from Microchip Technology, Inc. ofChandler, Ariz. The microcontroller 32 is powered by a 3.5 volt battery34. Attached to the enable line 36 of the microcontroller 32 is avoltage regulator 39 for regulating the input voltage to themicrocontroller 32. Attached to the clock line 38 and the data line 40is a charge transfer sensor 42. The data line 40 transmits data from thecharge transfer sensor 42 to the microcontroller indicting the distanceof an object, in this case a person's buttocks, from the sensor area 26.The data is preferably in the form of a hexadecimal numberrepresentative of the relative distance of the person from the sensorarea. In the preferred embodiment, the charge transfer sensor 42 is aQProx QT117 available from Quantum Research of Hamble, Southampton,United Kingdom. A ground line 44 is also connected to the chargetransfer sensor 42, as well as to the grounding plane 28. The capacitivesensor 42 also requires a capacitor 46, having a capacitance C_(s),attached to two lines of the sensor 42. The capacitance of the capacitor46 is preferably 0.022 μF and a temperature stable dielectric such asCOG, but such value will change based upon the size and the applicationof the sensor.

Also attached to the microcontroller 32 are various outputs to alarmsand indicators 48, inputs from an on/off switch 50 and an operator inputswitch 52, and inputs from other controls 54, such as if the circuit 30is used as a feedback loop to automatically control the inflation of thecushion, as described below.

Referring to FIG. 4, the charge transfer sensor 42 employs a short, lowduty cycle burst of charge-transfer cycles with a burst controller 58and amplifier 62 to acquire its signal. Internally the signals aredigitally processed with an analog to digital converter (ADC) 60 togenerate the required output signals. The charge transfer sensor 42switches and charge measurement hardware functions are all internal tothe sensor 42. The ADC 60 is 14-bit single-slope switched capacitor ADCincluding both the required sensor 42 charge and transfer switches in aconfiguration that provides direct ADC conversion. The burst length isinversely proportional to the rate of charge buildup on the capacitor 46(C_(s)), which in turn depends on the values of C_(s), C_(x) (the loadcapacitance of the sensor) and V_(cc). V_(cc) is used as the chargereference voltage. Larger values of C_(x) cause the charge transferredinto C_(s) to accumulate more rapidly. As a result, the values of C_(s),C_(x) and V_(cc) should be fairly stable over the expected operatingtemperature range.

The internal ADC 60 treats C_(s) as a floating transfer capacitor. As adirect result, the sensor 26 can be connected to either SNS1 or SNS2with no performance difference. The polarity of the charge buildupacross C_(s) during a burst is the same in either case. C_(s) must be ofwithin a certain range for proper operation. It is important to limitthe amount of stray capacitance on both terminals, especially if theload C_(x) is already large, for example by minimizing trace and wirelengths and widths so as not to exceed the C_(x) load specification andto allow for a larger sensing electrode size if so desired. The circuitboard traces, wiring, and any components associated with or in contactwith SNS1 and SNS2 will become proximity sensitive and should be treatedwith caution.

The microcontroller 32 operates according to the flow chart of FIG. 5.In a first step, the device is powered on 100 and enters a continuouslymonitoring state 102. From this state, the microcontroller 32 monitorswhether an input operator input switch 52 has been depressed in decisionstep 104. If it is has not, the microcontroller 32 returns to themonitoring state 102. If the switch 52 has been depressed, the next stepis to determine whether the depression was for three seconds or less indecision step 106. If for three seconds or less, the battery health ischecked in step 108 and a present reading of the distance of the personfrom the sensor area 26 is determined in step 110.

If a button 52 is determined to have been pressed greater than threeseconds in step 106, then in step 112, the microcontroller 32 causes analarm 48 to beep momentarily and proceeds to step 114 where the circuitagain determines of the button 52 has been depressed for more than threemore seconds. If so, the microcontroller 32 cycles through a series offive sensitivity settings as indicated to the user by a rapid successionof beeps of the alarm 48 in step 116. The sensitivity setting is thenstored in step 118 and the circuit continues to step 110 to read thepresent distance.

If in step 114 it is determined that the button 52 has not beendepressed for an additional three seconds, a value indicating thepresent distance is stored as the preferred set point in step 120, andthe circuit sounds an alarm and continues to step 110 to read thepresent distance.

If in step 110, the present value of the distance of the person from thesensor area 26 is not readable, the circuit continues to step 122 andflashes yellow and red LEDs alternatively. If the value is readable, themicrocontroller 32 continues to step 124 and sets a tolerance above andbelow the current setpoint which will be considered within acceptablerange from the setpoint. Next, in step 126, the microcontroller 32decides whether the present reading is within range or above or belowrange.

If the reading is above range, in step 128, the microcontroller 32determines whether the current reading is greater than or equal to twocounts over the previously chosen and stored sensitivity plus thesetpoint. If the condition is true, the microcontroller 32 proceeds tostep 130 where the microcontroller 32 determines it is not presentlybeing used and goes to sleep until a reading is in the normal range. Ifthe condition is not true, the microcontroller 32 flashes a yellow LED48 to indicate that the cushion is overinflated. In either event, themicrocontroller 32 next optionally proceeds to step 134, where it logsthe current condition date and time. If the embodiment is not one inwhich the data indicating inflation status is logged, themicrocontroller will proceed to step 136.

In step 136, if the current reading is below the acceptable range, themicrocontroller will flash the red LED 48 and sound an audible alarm 48to indicate under inflation if the current reading is the secondconsecutive reading to determine under inflation and proceeds to step134.

After step 134, the microcontroller 32 determines whether a user haspushed the button 52 to silence the audible alarm 48 in step 138. Ifyes, the microcontroller 32 proceeds to step 140 and disables theaudible alarm 48 until a second button push or a current sensor readingshows a reading with the acceptable range. After steps 138 and 140, themicrocontroller 32 proceeds to step 102.

If it is determined in step 126 that the setpoint is within theacceptable range, the microcontroller 32 continues to step 142 where themicrocontroller 32 determines if the present reading was initiated by abutton 52 press. If yes, in step 144 the green LED 48 is flashed and themicrocontroller 32 returns to the monitoring state in step 102. If no,in step 146, the microcontroller 32 reinstates the timer and return tostep 102.

Returning back to step 102, if in the monitoring state ten minutes haveelapsed, the microcontroller 32 will initiate a current readingautomatically by proceeding to step 148 by performing a battery checkand proceeding to step 110.

As another example shown in FIG. 6, the sensor can be used in a hospitalbed to determine whether a patient has bottomed out when using aninflatable air mattress. In this instance, the bed comprises a bed frame200 comprising a spring support 202. Placed upon the spring support area shield plane 204 and a sensor plane 206. Upon the sensor plane 206 isplaced an air mattress 208. The shield plane 204 acts to isolate themetallic items of the bed 200, particularly the spring support 202, fromthe sensor plane 206. The sensor plane 206 in its simplest applicationcomprises a single sheet of conductive material, as with the previouslydiscussed embodiment. The driven shield isolates the metal items of abed and chair below the sensor plane 206. In a device without a drivenshield the effect of surrounding metal is subtracted by the usercreating a set-point based on the desired immersion level and therelative reading observed at that immersion.

Just as with the wheelchair cushion proximity detector, the circuitry 30operates in the same manner except that the shield plane 204 is drivento provide isolation from the metallic structure of the bed. Thedistance between the sensor plane 206 and the shield plane 204 ispreferably about ⅛″ to about ⅜″. A problem posed by the hospital bedsituation is the amount of metal in the bed and mattress supportstructure. The driven shield under the sensor or sensor area in the caseof multiplexed units (described below) shields the sensor plane 206 inthat direction of the location of the shield plane 204 giving increasedsensitivity in the desired direction and ignoring changes in conductivematerials and noise generating devices with position changes of therelative position of the device with the bed or other devices.

In this regard and referring to FIG. 7, the original circuit 30 ismodified to form circuit 30′. The numerals of circuit 30′ thatcorrespond to circuit 30 are unchanged. However, the circuit 30 furthercomprises an amplifier 302 which is driven from an output of the chargetransfer sensor 42 and serves to drive the shield plane 204 to isolatethe sensor plane 206 from the metal portions of the bed 200.

In another embodiment shown in FIG. 8, the bed 200 may be equipped withmultiple sensors 400-414 in the sensor plane 206. For example, the firstsensor 400 would be placed in the area of the patient's head, two moresensors 402 and 404 in the area of a patient's shoulders, yet anothersensor 406 in the area of the patient's buttocks, and finally two moresensors 408 and 410 in the area of the patient's feet. Entrapmentsensors 412 and 414 are also located near the bed rails to provide anindication that the patient has rolled to one side of the bed and haspossibly become entrapped in the railing.

The sensors 400-414 are all conductively attached to a charge transfersensor to form a single sensor plane 206. The shield plane 204 issimilarly divided into portions that correspond to the size and theshape of the sensors 400-414. The result is that one charge transfersensor 42 is required for each sensor 400-414.

To provide the ability to monitor an even greater number of sensors, acircuit 30″ as shown FIG. 9 can be implemented. The circuit is identicalto the circuit 30′ except that a multiplexer 500 is inserted between theoutput of the charger transfer sensor 204 and a plurality of sensors206, 206′ and 206″. The multiplexer 500 switches from sensor 206 tosensor 206′ to sensor 206″, in turn, in order to determine the distanceof the relevant portion of the lying person from the sensors 206, 206′,206″. In this manner, only one circuit 30″ is required to poll amultiplicity of sensors 400-414. Because of timing limitations ofavailable charge transfer sensors, a limited number of sensors can bedaisy chained. Also, due to stray capacitance issues the number ofsensors that can be reasonably multiplexed, a combination of multiplexedand daisy chained sensors may be implemented in order to maximize thenumber of sensors. Thus, for example, sixty-four sensors may beimplemented by arranging the sensors as eight daisy chains of sensorsmultiplexed to the circuits 30″ with each chain having eight sensors420, as shown in FIG. 17.

In that regard and referring to FIG. 10, an embodiment is shown whereinthirteen sensors 500-524 are provided which determine the patient'simmersion within the air cushion and two more sensors 526 and 528 areprovided that determine whether the patient has become entrapped in thebed rails. These sensors 500-528 may be either daisy chained, attachedto their own circuits or multiplexed. Moreover, a combination of daisychaining sensors and multiplexing sensors may be performed.

In FIG. 11, yet another embodiment is shown wherein the coverage area ofthe bed is higher, but with fewer sensors 600-610. This arrangement maybe more appropriate for monitoring not whether a person is properlyimmersed, but rather if they are present or absent from their bed. Suchan application would be useful in hospitals and nursing homes. Again,these sensors 600-610 may be either attached to their own circuits ormultiplexed.

Another application for the present invention defined in the claims isfor use as a feedback loop in the auto-inflation or auto-deflation of acushion for wheelchair. Referring to FIG. 12, such an embodiment isshown. Specifically, an output of the microcontroller 32 notifies avalve 700 to change positions to add air, release air or remain closedbased upon the inflation status of the cushion 10. The valve 700 isattached to a source of compressed air 702, which supplies compressedair when an under-inflation status is detected. Likewise, when anover-inflation status is detected the valve 700 slowly releases air fromthe cushion 10 until the proper inflation level is achieved. Similarly,in a low air loss cushion for a hospital bed the circuit may similarlyserve as a feedback loop to control mattress inflation, such as byproviding feedback to a bed blower control.

In the embodiments shown above, it is necessary to manually “teach” themicroprocessor the extents of the travel by indicating themicroprocessor the extents of proximity of the detected object. In thatmanner, the microprocessor can determine a relative proximity of thedetected object within the known range. In the embodiment of FIG. 13,the device may comprise a sensor within a sensor.

In this embodiment, there is provided a first sensor 800 comprising alarge area with respect to a second, smaller sensor 802. In theembodiment of FIG. 13, the second, smaller sensor 802 is surrounded bythe first, larger sensor 800. Below the first and second sensors 800 and802, and electrically isolated therefrom, is a ground plane 804 and adriven shield

The first sensor 800 is made fairly large to anticipate contact pointsover a surface of interest (for example, the area under a person'sbuttocks in a wheelchair cushion application). The large sensor 800gives a reading of charge transfer that is highly dependant on the sizeof the individual above the sensor. As a result, without manuallysetting the range of extents of travel of the person in the wheelchaircushion example, it is difficult to determine the precise proximity of aperson of unknown size.

Merely by way of example, a large person may range between a value of 76and 120 at the extents of travel of that person's proximity to thesensor 800. A small person may range between values of 100 and 150 attheir extents of proximity. Therefore, at the closest extent of travel,a large person may show a reading of 76 and the small person may show areading of 100 making it difficult to determine the proximity of aperson of unknown size.

However, the charge transfer of only the small sensor 802 is not asdependent on the size of the person above of the sensor. This is becausethe area of the sensor is small in relation to the person above thesensor. Unfortunately, however, the small sensor 802 cannot monitor alarge area of interest.

In the embodiment of FIG. 13, the multiplexer or switch 806 (FIG. 15),for example a single pole double throw analog switch such as the FSA3157available from Fairchild Semiconductor of South Portland, Me., is usedto alternately electrically connect the charge transfer sensor 42 toeither the small sensor 802 or to both the large sensor 800 and thesmall sensor 802. The microcontroller 32 may then read the proximityvalue of the small sensor 802 and determine, over the small area, therelative proximity of the object above. Next, the large sensor 800 andthe small sensor 802 are electrically connected to the charge transfersensor 42 and the proximity value of the object of interest will bedetermined. By correlating this value to the value determined by thesmall sensor 802, the range of values of proximity for the large sensor800 and small sensor 802 together can be determined based upon thepresent value for the small sensor 802. Alternatively, rather than usingthe value of the small sensor 802 to correlate with the value of thelarge sensor 800 and small sensor 802 together, the value of the largesensor 800 alone could be detected and correlated with the value smallsensor 802 to obtain a proximity value over only the large sensor's 800area.

Additionally, when sensing the proximity value of the small sensor 802,it is desirable to electrically connect the large sensor 800 to theground plane 804. This is accomplished by using a control line from themicrocontroller that controls the switch 806 and connects the peripheralsensor area either ground or part of the sensor. Alternatively, this mayalso be accomplished by utilizing the frame output of the chargetransfer device to make a logic switch after the first reading each timethe device is powered up.

While the embodiment of FIGS. 13 and 14 is shown having a driven shieldand a ground plane, it will be appreciated by one of ordinary skill inthe art that an embodiment not having the driven shield may also beimplemented without departing from the scope of the present invention.

Referring to FIG. 15, another embodiment of the present inventionprovides a visual display for graphically representing a relativeproximity value for a sensor or group of sensors. In this embodiment,the sensor array and its associated microcontroller 32 of FIG. 14 (shownin FIG. 15 as reference numeral 900) is electrically connected to areader device 902, which comprises a circuit board that provides aninterface between the sensors and microcontroller 32 and a displaydevice 904, which in the preferred embodiment is a computer. The readerdevice 902 preferably connects to the display device 904 via a USB cable906. The display device 904 runs a program which continuously reads thedigital value of each sensor in the array, and represents those valuesgraphically. The reader device 902 is not required to be a separateunit. Its functionality could be incorporated into either the sensorcircuit or the display device 904.

Because the sensors are not calibrated, and because the actual digitalvalue for a particular proximity level is influenced by a number offactors (such as sensor size, shape, and material, and mattress orcushion density and thickness), the display device 904 should provide amethod of correlating the actual digital values with proximity levelsfor each sensor, for each particular system. For example, it can providea table of maximum and minimum values for each sensor. The maximum valueis set to the actual digital value that results from a proximity levelof infinity (a body in farthest proximity), and the minimum value is setto the actual value that results from a proximity level of zero (a bodyin nearest proximity). Then, the digital values within the maximum andminimum range are translated and displayed more meaningfully asproximity values. These values are determined and entered manually, orby way of an auto-range mode in the display device. In this mode, itwould monitor the digital values for each sensor, and automaticallyadjust the table entries as it observes new maximum and minimum values,and as a technician provides appropriate near and far stimulus to eachsensor.

While the invention is described above as separate devices used inconjunction with a hospital bed or wheelchair cover, the devices may beintegrally formed with the wheelchair cushion or hospital mattress orwith the wheelchair or hospital bed without departing from the scope ofthe present invention.

Other applications for the proximity sensor would be as a bed/chairoccupancy detector to notify hospital or nursing home attendants as tothe presence or absence of the patients from a bed or chair. Similarly,it could serve as a toilet seat occupancy device for notifying when adisabled patient has been left on a toilet seat for too long. Moreover,it may be used for car seat occupancy detection to control air bagdeployment in a crash. Another application would be for seat occupancydetection on an airplane.

There are several veterinary applications for the invention as well. Forexample, before giving birth horses will lay down in their stall. Horsebreeders will typically keep a close eye on a horse about to give birth.In order to ease the burden of checking on the horse, a sensor can beplaced in the floor of the stall. When the animal lies down, the breederwould be notified by the circuit to attend to the horse. Additionally,it could be used in horse trailers to monitor the horse.

It could similarly be used on a person as a geriatric fall monitor. Thesensor would be placed on the person's body and when proximity with thefloor was detected, an alarm for help automatically sounded. Possiblelocations would be on the person's hip or shoulder.

Finally, if the conductive layer were placed in close proximity contactwith the torso, it could be used to monitor patient vital signs, such asrespiration and heartbeat.

The above examples show that the invention, as defined by the claims,has far ranging application and should not be limited merely to theembodiments shown and described in detail. Instead the invention shouldbe limited only to the explicit words of the claims, and the claimsshould not be arbitrarily limited to embodiments shown in thespecification. The scope of protection is only limited by the scope ofthe accompanying claims, and the Examiner should examine the claims onthat basis.

1. An immersion sensor assembly for use with a cushion or mattress formeasuring the depth of immersion of a person within the cushion ormattress comprising: a sensor comprising a conductive material; a groundcomprising a second conductive material electrically isolated from thesensor and at least partially surrounding the sensor, the groundelectrically connected to and maintained at a ground electricalpotential; a circuit comprising a microprocessor and a referencecapacitor, the circuit adapted to send short bursts of electricalcurrent to the sensor and the reference capacitor, the circuit adaptedto measure the length of time the burst of current takes to charge thereference capacitor and the circuit adapted to calculate the immersionof the person into the cushion or mattress based upon the time taken tocharge the reference capacitor; and wherein the sensor and the groundwhen the immersion sensor is laid flat, are nominally coplanar.
 2. Theimmersion sensor assembly of claim 1 further comprising at least onelayer of nonconductive material disposed over the sensor and the ground.3. The immersion sensor assembly of claim 2 wherein the nonconductivematerial is foam.
 4. The immersion sensor assembly of claim 1 whereinthe sensor is disposed within a protective encasement of neoprenerubber.
 5. The immersion sensor assembly of claim 1 wherein the circuitcomprises a capacitive sensor.
 6. The immersion sensor assembly of claim1 further comprising a driven shield to isolate the sensor from theeffect of electromagnetic interference or metallic objects in thedirection of the driven shield from the sensor.
 7. The immersion sensorassembly of claim 1 comprising a plurality of sensors and a plurality ofgrounds.
 8. The immersion sensor assembly of claim 7 wherein the circuitis attached to two or more of the plurality of sensors to simultaneouslydetermine the proximity of people to each of the sensors.
 9. Theimmersion sensor assembly of claim 8 wherein the circuit is adapted toobtain a proximity measurement from a second sensor to determine a scaleof proximity for proximity measurements from a first sensor.
 10. Theimmersion sensor assembly of claim 7 wherein the circuit comprises amultiplexer, the multiplexer being attached to two or more of thesensors and also to the circuit and adapted to selectively connect thesensors to the circuit.
 11. An immersion sensor assembly comprising: acushion or mattress having an adjustable depth of immersion of a personon the cushion or mattress; a sensor comprising a sheet of conductivematerial; a grounding plane being in the area around the sensor andallowing a capacitance measurement to be made relative to the distancebetween a person and the sensor; a shield comprising a second sheet ofconductive material located adjacent to and having generally the sameshape and area as the sensor; a circuit comprising a referencecapacitor, the circuit adapted to send short bursts of electricalcurrent to the sensor and the reference capacitor, measure the length oftime the burst of current takes to charge the reference capacitor andthe circuit adapted to calculate the relative proximity of the personbased upon the time taken to charge the reference capacitor, drive theshield to electrically isolate the shield from external capacitance orelectromagnetic interference, and provide an indication when the personis too deeply immersed within the cushion or mattress.
 12. A bedassembly for the support of an individual positioned on the bed, the bedassembly comprising: a bed frame, said bed frame comprising metalcomponents; a mattress on the bed; and a sensor operatively associatedwith the mattress, the sensor comprising, a microcontroller, a groundlayer surrounding the sensor; a shield layer comprising a sheet ofconductive material and driving the shield to isolate the sensor fromthe effect of stray capacitance and electromagnetic interference fromthe metal bed frame components; a circuit for detecting the capacitiveeffect of the individual on the mattress and providing a relative outputfor the microcontroller; wherein the planes formed by the sensor and theshield, when the immersion sensor is laid flat, are nominally parallel;and wherein the circuit is adapted to provide an indication when theindividual is too deeply immersed within the mattress or when theindividual is off the mattress.
 13. The bed assembly of claim 12 whereinthe sensor further comprises at least one layer of nonconductivematerial disposed over the sensor.
 14. The bed assembly of claim 12wherein the sensor is integrally formed with a mattress.
 15. The bedassembly of claim 12 wherein the sensor comprises a plurality of sensorsand the shield is sized to isolate the plurality of sensors.
 16. The bedassembly of claim 15 wherein the circuit comprises a multiplexer, themultiplexer being attached to two or more of the sensors and also to thecircuit and adapted to selectively connect the sensors to the circuit.17. In a bed assembly comprising a bed frame and a mattress, a method ofdetermining the proper immersion of a person within the mattresscomprising the steps of: providing a first sensor below mattresscomprising a sheet of conductive material; providing a ground layercomprising a sheet of conductive material that is nominally coplanar andmaintained at a ground potential below the cushion or mattress;providing a shield layer comprising a sheet of conductive material anddriving the shield to isolate the sensor from the effect of straycapacitance and electromagnetic interference from the bed frame. sendingshort bursts of electrical current to the sensor and a capacitor;measuring the length of time the burst of current takes to charge thecapacitor; calculating the proximity of the person based upon the timetaken to charge the capacitor; and providing an indication when theperson is either over-immersed or under-immersed within mattress. 18.The method of claim 17 further comprising the steps of: providing asecond sensor of smaller area than the first sensor comprising a secondsheet of conductive material; sending short bursts of electrical currentto the second sensor and the capacitor; measuring the length of time theburst of current takes to charge the capacitor; and calculating theproximity of the person based upon the time taken to charge thecapacitor; and determining a scale for the calculation from the firstsensor from the calculation obtained from the second sensor.